USB: xhci: Allow allocation of commands without input contexts.

The xhci_command structure is the basic structure for issuing commands to
the xHCI hardware.  It contains a struct completion (so that the issuing
function can wait on the command), command status, and a input context
that is used to pass information to the hardware.  Not all commands need
the input context, so make it optional to allocate.  Allow
xhci_free_container_ctx() to be passed a NULL input context, to make
freeing the xhci_command structure simple.

Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>